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Function Block Model


A unique characteristic of the Foundation architecture ensuring device interoperability is its use of a fully specified, standard User Layer based on "Blocks" and Device Descriptions (DDs). The User Layer defines a Function Block Application Process (FBAP) using Resource Blocks, Function Blocks, Transducer Blocks, System Management, Network Management and DD technology.

Blocks enable fieldbus device functionality

Flexible Function Blocks enhance control in the field



Blocks enable fieldbus device functionality

Resource Blocks define parameters that pertain to the entire application process (e.g., manufacturing ID, device type, etc.). Function Blocks encapsulate control functions (e.g., PID controller, analog input, etc.) and Transducer Blocks offer an interface to sensors such as temperature, pressure and flow, and actuators for valves.


Blocks enable fieldbus device functionality

Blocks are incorporated into fieldbus devices to achieve the desired device functionality, as well as to define a wide range of features and behaviors that must work in a standard way for devices to interoperate. For example, a simple temperature transmitter may contain an Analog Input (AI) function block. A control valve might contain a Proportional/Integral/Derivative (PID) function block as well as the expected Analog Output (AO) block. Thus, a complete control loop can be built using only a simple transmitter and a control valve.

Each block's parameters are represented by object descriptions that define how the parameters are communicated on the fieldbus network. The FBAP represents a very comprehensive application model that, in conjunction with the protocol, provides the technology that allows devices from different manufacturers to interoperate.

As specified by the FBAP, a fieldbus device must have a Resource Block and at least one Function Block with input and/or output parameters that link to other function blocks, either in the same device or in separate devices by means of the bus. The "link object" defines the connection. Each input/output parameter passed will have a value and a status. The status portion of each parameter carries information on the quality of each value, of good, uncertain or bad.

In addition, the FBAP specifies the handling of control modes, alarms, events, trend reports and views. All of these features must comply with the Foundation specification in order for a device to be considered interoperable at the User Layer.

Distribution of control to the field device is made possible by synchronization of function block execution and communication of function block parameters on the fieldbus. This function, along with publication of the time of day to all devices, automatic switch over to a redundant time publisher, assignment of device addresses, and searching for parameter names or "tags" on the fieldbus, are handled by System Management (SM) and Network Management (NM).


Flexible Function Blocks enhance control in the field

A Flexible Function Block (FFB) is a user-defined block. The FFB allows a manufacturer or user to define block parameters and algorithms to suit an application that interoperates with standard function blocks and host systems.

The use of FFBs enhances the ability to move control functionality to the field-a unique and important characteristic of Foundation technology. This strategy eliminates the need for proprietary unit controllers in many applications, and in doing so, decreases equipment costs and space requirements. Distributing control to the field also minimizes the effects of any loss or failure of supervisory control.




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